Tracking logs

`feedback.shown` is not implemented, since it is already implied by a `problem.attempted` event. There is no other way to trigger a feedback.

It would be nice to add an `item_id` to the `feedback.closed` event. It might be a good idea, and relatively easy, but that would include giving IDs for each feedback, meaning some extra work and code.


I think I'd be tempted to push this into xblock core. We want to encourage XBlock developers to publish events, so making it easy to do from the front-end would be very good.

@mulby: Have you guys thought about this at all?